RTK Fixes Under a Hardwood Canopy
A surveyor is mapping a 12-acre wooded ridge in Rowan County for a boundary line and a building pad. The project requires 0.10-ft relative accuracy between property corners. The surveyor uses network RTK GNSS with 12 satellites and PDOP 2.8. In the open, repeated shots on two NGS monuments in Kentucky Single Zone agree within 0.03 ft. Under the canopy, repeated shots on the same property corner vary by 0.09 to 0.14 ft, and check shots on the two NGS monuments show residuals of 0.11 ft and 0.14 ft. A total station traverse from two open-sky control points closes 0.03 ft in 1,850 ft. The client wants the boundary corners staked the same day, and the contractor wants to start clearing. The surveyor has enough time to run a total station traverse but not to return for a static GNSS session.
1. Does the RTK data alone meet the required accuracy for the property corners?
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No. The open-sky check shots are good, but the canopy shots are not. The repeatability of 0.09 to 0.14 ft and the independent check residuals of 0.11 and 0.14 ft exceed the 0.10-ft relative accuracy requirement. A single RTK fix type, even with 12 satellites and PDOP 2.8, is not proof of accuracy; PDOP and satellite count describe geometry, not multipath or canopy-induced error. The surveyor should compare the observed residuals to the project tolerance and to the manufacturer's stated precision, and should treat the canopy results as failing the contract requirement. The total station traverse closure of 0.03 ft in 1,850 ft (about 1:61,000) is much stronger evidence for the boundary corners.
2. What field procedure should the surveyor use to complete the boundary corners?
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The surveyor should use the RTK-derived open-sky control as a starting point only if it is independently checked, then run a total station traverse from those control points to the canopy boundary corners. The traverse should close on another control point or return to the start, with angular and linear closure checked. If the total station cannot see between corners, the surveyor should use short side shots from traverse stations, repeat measurements, and check the final corner positions by an independent method. The surveyor should not simply average many RTK shots under canopy; averaging reduces random noise but does not remove multipath bias. If static GNSS is not possible, the total station traverse is the appropriate backup. The surveyor should also verify the Kentucky Single Zone scale factor and elevation factor before reducing distances to grid.
3. How should the surveyor document the accuracy decision?
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The field notes and report should record the date, time, satellite geometry, PDOP, fix type, number of epochs, and the exact check-shot residuals on the NGS monuments. The surveyor should state that the canopy RTK did not meet the 0.10-ft relative accuracy requirement and that the boundary corners were established by total station traverse from checked control. The traverse closure, angular misclosure, and any adjusted coordinates should be included. The plat or report should identify the source of each corner and the accuracy achieved. If a client or contractor pressures the surveyor to release the RTK-only corners, the surveyor should explain the documented deficiency and decline to certify them. Good documentation shows the judgment was based on evidence, not on convenience.